DESCRIPTION Passed a format string - eg ltd it will try
and find a suitable id. It scans list of devices to build up
a free map, then chooses the first empty slot. The caller
must hold the dev_base or rtnl lock while allocating the
name and adding the device in order to avoid duplicates.
Limited to bits_per_byte * page size devices (ie 32K on most
platforms). Returns the number of the unit assigned or a
negative errno code.